1. Field of the Invention
The present invention relates to a drive circuit of a display device, and more particularly to a shift register that is adaptive for preventing malfunction and damage, and a driving method thereof.
2. Description of the Related Art
Recently, there have been developed various flat panel display devices that can reduce their weight and size which is an advantage over a cathode ray tube CRT. The flat panel display device includes a liquid crystal display LCD, a field effect display FED, a plasma display panel PDP, an electro luminescence EL display device and so on.
The liquid crystal display (hereinafter, referred to as “LCD”) device among the flat panel display devices has its application range on a broadening trend owing to its characteristics such as lightness, slimness, low power consumption and so on. According to such a trend, the LCD has been used in office automation equipment, audio and video equipment and so on.
The related art liquid crystal display device controls the light transmissivity of liquid crystal by use of an electric field, thereby displaying a picture. For this, the liquid crystal display device includes a liquid crystal display panel where liquid crystal cells are arranged in a matrix shape with a drive circuit to drive the liquid crystal display panel.
The liquid crystal display panel has gate lines and data lines arranged to cross each other and the liquid crystal cells are located at areas provided by the crossing of the gate lines and the data lines. There are provided pixel electrodes and a common electrode in the liquid crystal display panel for applying the electric field to each of the liquid crystal cells. The pixel electrode is connected to a thin film transistor, which is a switching device. The thin film transistor supplies video data to the pixel electrode in accordance with signals of a data driver and a gate driver.
The drive circuit includes the gate driver to drive the gate lines and the data driver to drive the data lines. The gate driver sequentially supplies scan signals to the gate lines to sequentially drive the liquid crystal cells on the liquid crystal display panel line by line. The data driver supplies video signals to each of the data lines whenever the gate signal is supplied to any one of the gate lines. Accordingly, the liquid crystal display device controls the light transmissivity by the electric field applied between the pixel electrode and the common electrode in accordance with the video signal by liquid crystal cells, thereby displaying a picture.
The gate lines, which are horizontal lines of the liquid crystal matrix, are selected by the gate signal supplied from a shift register circuit.
FIG. 1 is a diagram briefly representing shift registers of a gate driver, and FIG. 2 is a diagram representing gate signals applied to each of gate lines. The gate driver includes a clock signal line for supplying a clock signal from a clock signal generating source, and the gate lines and shift registers SR1 to SRn for supplying the gate signals.
The shift register SR1 to SRn is synchronized with the clock signal to supply the gate signal to the gate line g1 to gn for selecting the liquid crystal cells by lines. For this, the shift register SR1 to SRn is connected to the gate line and the clock signal line. A start pulse (Start) (not shown) is supplied to the first shift register of the shift registers for generating the gate signal, and the output terminal of each shift register is connected to the input terminal of the next stage. Hereby, the output of the previous stage shift register is used as a start pulse of the next stage shift register.
The start pulse and the clock signal cause the gate signal supplied to the gate line g1 to gn to be shifted by gate lines g1 to gn, thereby supplying the shifted gate signal. And through this, the liquid crystal cell line is selected by lines.
FIG. 3 is a diagram briefly representing a drive waveform to drive the shift register SR of FIG. 1.
FIG. 3 shows signal waves of a gate output terminal Vg, a first clock signal line CLK1, a first output terminal Q and a second output terminal Qb of a switching signal generator. If the first output terminal Q has a high (high or 1) logic value and the second output terminal Qb has a low (low or 0) logic value, then the clock signal from the first clock signal line CLK1 is outputted through the gate output terminal Vg. On the contrary, if the first output terminal Q has a low logic value and the second output terminal Qb has a high logic value, then the clock signal is not outputted to the gate output terminal Vg even though the clock signal of the first clock signal line CLK1 has a high logic value. The period when the signal is supplied to the gate line gn through the gate output terminal Vg is very short in comparison with the period when there is no output to the gate output terminal Vg. In other words, for most of the period (about 90%), the second output terminal Qb has the high logic value. This is because it is required for the gate signal to have the low logic value.
However, if a high voltage is continuously applied to the gate terminal of the thin film transistor, i.e., the second output terminal Qb, the threshold voltage of the thin film transistor increases to make the mobility grow worse.
FIG. 4 is a diagram representing an increase of threshold voltage in accordance with the lapse of time.
Referring to FIG. 4, the vertical axis of coordinates represents the threshold voltage of the thin film transistor and the horizontal axis represents the lapse of time. In order to maintain the output of the gate output terminal Vg low, if a voltage is continuously applied to the second output terminal Qb, the threshold voltage increases as shown in FIG. 4. In this way, if the threshold voltage increases due to long time use, the thin film transistor is continuously deteriorated. In other words, if the voltage is continuously applied to the second output terminal Qb, there is a problem in that damage of the device or malfunction by the deterioration of the thin film transistor is generated.